logical keyboard sensor brick specificity
Requirements

The issues is that you have an ambiguous description at the first place. The sensors do not (and should not) know of each other. They measure a specific event.

This means when you let a sensor measure for A it will measure exactly that regardless what other keys are pressed (as keys are independent from each other).

The keyboard sensor allows to sense the press of several keys at the same time. All of that keys needs to be pressed. This is - as in your situation - not in all situations what you want.

Controllers

To get a more complex setup the controllers will help you.

You can combine sensor input in a logical way to get an activation or deactivation signal to the actuator.


if A+left Shift then activate X.
if A+not left Shift then activate Y.
if not A then activate Z.


Typically the "not" is more complicated. 

The following examples assume A requests "move" while left Shift+A requests to "move turbo".

Inverted Sensor



You use a second sensor that inverts the output (keyboard sensor on  left Shift  with [invert] enabled).

This allows to use a simple controller e.g. AND

Drawback: 


you have to configure the sensing twice (one inverted and one not inverted). 


Expression controller

This allows you much more complex logic expressions. 

Drawbacks: 


You need to type rather than click. 
Renaming the sensor will break the expression.


You need two sensors measuring each key. 



Requesting "move":

The Expression controller measures turn and not turbo. This combines the two sensors (by name).   

Requesting "move turbo":

You could use an Expression controller as well with turn and turbo, but the AND controller already does that. So it is cheaper with an AND controller.

Python Controller

You can use a Python controller to do the logical combination. But this is like buying a car factory just to get a car. It is possible, but for this purposes not worth.

State Machine

A complete different concept is the use of a state machine. Luckily there is one already build in. 

You can define "move" and "move turbo" as two states plus one without moving: "waiting". You transit between the states via (sensors). 

The advantage is that you do not need to worry the keys of the other states. You can even reuse keys (e.g. A means different things in different states).

But I think this does not really fit into your situation. So I will not explain it in more detail.